Joshua Kenneth Robertson Tabh , Andreas Nord

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species persistence on our planet. Among the most well-known ecogeographical rules are James’ rule (an intraspecific variant of Bergmann’s rule) and Allen’s rule, with each correlating ambient temperature to the size and shape of endotherms within a species. These two rules have recently gained renewed research attention, largely with the goal of understanding how they emerge (e.g. via natural selection or phenotypic plasticity), and thus, whether they may emerge quickly enough to facilitate species persistence in a warming world. Yet despite this attention, the precise proximate and ultimate drivers of James’ and Allen’s rules remain unresolved. In this paper, we review over a century of empirical literature surrounding these rules and ask whether each could be explained by plastic effects of developmental temperature on adult phenotype among endotherms. Across birds and mammals, studies strongly support developmental plasticity as a driver of James’ and Allen’s rules, particularly with regards to Allen’s rule and responses to heat. However, we find that plastic contributions toward each are non-linear and probably depend on: (1) efficiency of energy use at given ambient temperatures (James’ rule), and (2) thermal advantages at given ambient temperatures (Allen’s rule). These findings suggest that, among endotherms, rapid changes in body shape and size will continue to occur, but generalising the direction of responses across populations (e.g. as “shrinking” or “shape-shifting”) is likely naive.